deuterium vs. hydrogen

an isotope of hydrogen, having twice the mass of ordinary hydrogen; heavy hydrogen. : 2 H, D; : 2.01; : 1.

a colorless, odorless, flammable gas that combines chemically with oxygen to form water: the lightest of the known elements. : H; : 1.00797; : 1; density: 0.0899 grams/liter at 0°C and 760 millimeters pressure.